Both boron nitride and carbon have a soft hexagonal form that can be converted under high pressure to either of two hard forms: (1) a cubic form with a zincblende crystalline structure; and (2) a hexagonal form with a wurtzite crystalline structure. The specific gravities of these forms are 2.28 (soft), 3.49( hard, cubic, zincblende) and 3.49 (hard, hexagonal, wurtzite). These forms of boron nitride ("BN") are often referred to as low density BN, cubic BN (C-BN) and wurtzite BN (W-BN), respectively.
To produce high density BN, one can apply static or dynamic high pressures to low density BN and produce small aggregates of high density BN with diameters no higher than 100 microns (.mu.m). Many commercial applications of high density BNrequire aggregates of sizes much larger than 100 .mu.m.
Axial propagation of a shock wave through brittle, inorganic powders (.apprxeq.4500 .mu.m in diameter) is disclosed in U.S. Pat. No. 3,367,766, issued to Barrington and Bergmann. The materials used include alumina, barium ferrite, barium titinate, silicon carbide, boron carbide, magnesium oxide, titanium carbide and bismuth telluride. The shock wave is produced by detonation of an explosive contiguous with one end of a container for the powder to be aggregated.
A method of bonding together diamond powder particles, using shock waves, is disclosed by Dunnington in U.S. Pat. No. 3,399,254. The powder sample is confined in the interior of a hollow, flat slab or disk, and one broad, flat face of this container is impacted by an explosively-driven flyer plate or projectile. The shock wave pressures used here are limited to modest pressures (.apprxeq.300 kilobars), due to sample recovery problems.
Cowan and Holtzman, in U.S. Pat. No. 3,401,019, disclose a method for producing a shock wave of sufficient intensity to convert carbon to diamond initially, using a contiguous cooling medium that keeps the material. temperature of the shocked material below 2000.degree. C., preferably below 1800.degree. C. The cooling medium must have sufficient thermal conductivity that excessive graphitization does not occur after release of the shock wave pressures. This patent notes that a straightforward shock synthesis cannot produce satisfactory yields of diamond, due to excessive graphitization of the diamond. After release of the shock wave pressures, the diamond is initially hotter than the carbon from which it is formed. The inventors begin with carbon, preferably in graphite form, already compacted to about 75 percent. of the theoretical density for diamond, and apparently allow the chosen cooling medium to surround and fill the interstitial regions of the partly compacted graphite. This admixture is then subjected to a shock wave of at least 750 kilobars, preferably at least 1,000-2,000 kilobars. This approach begins with graphite and requires application of very high shock wave pressures to convert the starting material to diamond.
U.S. Pat. No. 3,568,248, issued to Cowan, discloses an end closure or plug for a cylindrical container of material that is to be subjected to a shock wave. The plug includes a first, substantially cylindrically shaped section in contact with the sample at a plug end along the longitudinal axis of the cylinder. The end plug material has a shock impedance (the product of initial material density and shock wave velocity in the material) equal to the shock impedance of the sample. A second section of the plug, in contact with the first section along the cylinder longitudinal axis, has the same shock impedance as the first section and is arranged to carry off most of the longitudinally propagating shock wave energy by spallation at an exposed end of this second section. In one embodiment, the first section has gradually decreasing porosity as one moves away from the sample toward the section section, and the second section has gradually increasing porosity as one continues in the same direction. This patent assumes that the shock wave will move primarily along the cylinder longitudinal axis.
Balchan and Cowan, in U.S. Pat. No. 3,667,911, disclose a method of shock wave treatment of a solid material, such as diamond, boron nitride or silicon carbide powder, by propagating a shock wave axially along the sample at substantially uniform velocity. The sample's physical extension in this wave propagation direction is much greater than the sample's physical extension in any transverse direction. The shock wave is generated (1) by impacting the sample at one end with an explosively driven impact plate or (2) by detonating a high explosive in contact with the sample at one end. The sample may be positioned in a container. Ideally, the shock wave is planar, with the defining plane being perpendicular to the axial direction of shock wave propagation, and the shock wave energy is sufficient that the associated wave pressure is substantially constant throughout this perpendicular plane. The sample's axial length should substantially exceed the distance ("start-up length") required to establish shock wave steady state conditions; the start-up length is approximately five times the transverse diameter of the sample. Alternatively, a solid material, having the same density, shock impedance and transverse diameter at the sample and having an axial length at least as large as the start-up length, should be provided at an axial location between, and in contact with both of, the sample and the explosive for application of the pressure pulse that produces the shock wave. Details of propagation of a shock wave in any direction differing substantially from the axial direction (e.g., in a radial direction) are not discussed, and such propagation would probably be inconsistent with application of this invention. This patent contains a good mathematical discussion of the generation and propagation of shock waves in a solid material.
A method of aggregating small, hard particles, such as diamond, into larger aggregates by passage of shock waves therethrough is disclosed by Balchan and Cowan in U.S. Pat. No. 3,851,027. The sample particles are dispersed interstitially in a carrier matrix having smaller porosity that has smaller porosity and larger post-shock deformability than the interstitial particles. The carrier matrix is usually formed as a slab or disk, and one broad, flat face of the matrix is impacted by an explosively-driven projectile or driver plate to produce a shock wave that travels through the carrier matrix/interstitial particles combination and bonds many of the interstitial particles together. It appears that the hard particles are intended to coalesce. into a plurality of larger size aggregates. Use of the carrier matrix apparently is intended to prevent aggregation of all the hard particles into a single mass. The hard particles that are bondable by this technique are asserted to include diamond, boron nitride, silicon carbide and silicon nitride. Pressures of 100 kilobars and higher from an axially propagating shock wave are used for this purpose.
Two U.S. Pat. Nos. 4,201,757 and 4,231,980, issued to Corrigan, disclose use of an explosively-driven flyer plate to generate shock waves in low density boron nitride to produce the high density wurtzite form of boron nitride. The impact of the flyer plate on one surface of the boron nitride is arranged to produce a shock wave in the longitudinal axis (C-axis) direction, as usual. The Corrigan patents assert that the shock compression "snaps" the low density boron nitride from a loosely packed crystalline form into a high density, close-packed form of boron nitride. The technique is apparently orientation dependent, because the inventor emphasizes that the shock wave must be directed along the C-axis of the soft form of the material. The shock wave pressure used in the Corrigan patents is 100-500 kilobars and is applied to the low density form of recrystallized pyrolytic boron nitride, rather than to the high density forms of the boron nitride. The size of boron nitride aggregates produced is about 100 .mu.m.
None of these patents discloses and seriously discusses generation and propagation of radial shock waves through a sample. Most of these patents begin with the sample material in powder form, not as small aggregates to be further aggregated into larger size aggregates. In particular, the techniques disclosed in these patents do not produce high density aggregates of size larger than about 100 .mu.m. What is needed is an approach that will produce high density aggregates of boron nitride with diameters up to 1 cm.